1. Field of the Invention
The present invention relates to an image reading apparatus which reads a color image, and a test chart for correcting color balance.
2. Description of the Related Art
Light from light sources such as a fluorescent lamp and a halogen lamp in a scanner for reading a draft has an emission intensity which is lower at opposite end portions than at a middle portion. Therefore, when a CCD sensor comprising four lines including monochromatic lines and lines for color is used as a photoelectric conversion element, there is a problem that brightness of the opposite end portions of the CCD sensor drops compared with the middle portion of the sensor. It is known that this is why shading correction is executed with respect to electronic data from the CCD sensor.
The line sensor for color produces electronic data from an output of each of red (R), green (G), blue (B) colors. However, the color balance of the light from the light source collapses with an elapse of time. Therefore, a balance in a range of a difference (MAX-MIN) between a maximum value and a minimum value of the output value of each color also sometimes collapses.
For example, it is known that when a xenon lamp is used in the light source, the lamp is degraded, and the emission balance collapses, blue emission degradation occurs, emitted light entirely becomes reddish, and the MAX value of the output value of each color changes, for example, like R: 5V, G: 3V, B: 1V.
The output from this CCD sensor is improved by the shading correction. However, since the balance of the ratio of the original output value range is bad, it has been difficult to suppress degradation of resolution only with the shading correction.
Moreover, this collapse of the color balance raises a problem that a color character is recognized even in the case where a black character draft is read.
The collapse of the color balance is sometimes caused by irregular reflection by dirt on a mirror or the like, and there is a problem that reflectance for each light having a different emission frequency is degraded.
Furthermore, when the problem of the color balance occurs, there is a problem of cost increase because optical members such as the lamp of the light source and the mirror are required to be changed. There is also a problem that a maintenance cycle of cleaning by a serviceman is shortened.
An object of the present invention is to provide an image reading apparatus including a photoelectric conversion element for color, which is capable of improving deterioration of resolution by collapse of color balance and preventing wrong recognition of color and extending life of an optical member.